Extruders are known having a pair of like screws tapered axially from an inlet end toward an outlet end. This taper is achieved by decreasing the minor or root diameter, a dimension equal to twice the radial distance between the central axis of the screw and the base of the root region between adjacent turns of flight of the screw thread, with the screw threads of the two screws being of constant radial depth from the inlet end to the outlet end of the extruder. Thus the difference between the major and minor diameters of the thread remains equal but the ratio between the thread depth, equal to half the difference between the major and minor diameters, and the minor or major diameter varies along the screw, decreasing from the output side toward the input side of the screws.
Added to this ratio variation is the considerable radially directed shear in the synthetic-resin mass being worked by the screws at the input end regions of the screws. It is necessary to maintain the axial compression of the plastified mass in the extruder within a close range because of the sensitivity to pressure of most molding operations. In addition this particular construction has the disadvantage that the synthetic-resin mass in the degassing zone has a volume which is relatively small. Since this particular volume is determinative of the output of the machine, any limitation on it limits the capacity of the extruder.
Whether the two screws are of like hand, and are, therefore, rotated in the same direction or of opposite hand and rotated in opposite directions, considerable disadvantages remain with this type of construction.